In hitherto known methods of electronically controlling the air-fuel ratio of air-fuel mixture supplied to an internal combustion engine, there is usually adopted a so-called "feedback control based on an O.sub.2 -sensor" (hereinafter referred to simply as O.sub.2 -control or O.sub.2 -feedback control in which a carburetor is controlled through a feedback loop on the basis of the quantity of oxygen contained in the engine exhaust gas and detected by an O.sub.2 -sensor. The conventional O.sub.2 -feedback control method however cannot assure the control of the air-fuel ratio with a high accuracy particularly in the transient operating states of the engine in which operating conditions of the engine undergo remarkable variations, because the O.sub.2 -feedback control has a rather poor response behavior due to an inherent delay in response.
Further, since a proportional plus integral control is performed with a predetermined gain in the O.sub.2 -feedback control, the air-fuel ratio will vary with a certain periodicity as a function of a periodical change in the control quantity, resulting in an unsmooth rotation of the engine, whereby the vehicle body is subjected to undesirable vibrations due to the involved hunting and surging, to a great disadvantage.
There has hitherto been also known an air-fuel ratio control method in which optimum control data (i.e. data for controlling the air-fuel ratio at an optimum) are previously prepared in dependence on corresponding parameters of the engine operation such as the number of revolutions the engine, negative pressure (degree of vacuum) in an intake manifold and so forth in the form of a data map (or data table), wherein the air-fuel ratio control is performed electronically by reading out relevant data from the data map. Hereinafter, this control is referred to as map control.